So there is a 3" difference @ 500 yards between the 6.5 and the .308, while the 6.5 uses less powder to do it and fits in an AR-15 platform. You can reform easily obtained and relatively cheap 7.62x39 brass to get it. It still has over 800 ft/lbs of energy at 500 yards, quite a bit more than the 6.8 or the 6x45. It's still supersonic past 1000, while the .308 is going transsonic around that point depending on load and bullet (I've shot quite a bit of M118 and M118LR at 1000, and out of gas guns to boot). It does this with a 120gr bullet (or 123, depending on who made it and what load it is) AND it isn't a magnum. Hmmmm.
The Grendel uses the 7.62x39 boltface, as was already mentioned. This is about .445. The .243 WSSM, by contrast, has a bolt face size of .535. So why, if the 7.62x39 bolt lugs and extractor are so "weak", would one want to shoot a more powerful cartridge that uses a much larger boltface, and by extension a bolt with a thinner margin, if you're worried about the extractor and bolt lugs on a 6.5 Grendel/7.62x39? And if either one is so weak that folks are worried about it, would manufacturers risk all sorts of product liability selling them?
So it still sounds to me like it depends on what you want, what you like, what you're going to do with it, and what platform you want to use it in.
(And don't get me wrong. I have an urge to get a .338 WSSM wildcat from DTech one of these days!)![]()